The present invention relates to a facile and utilitarian colorimetric test method, composition and device for the determination of specific gravity or total dissolved solids in low specific gravity aqueous samples. Because of its ability to measure low specific gravity fluids, it is primarily directed to potable and recreational water samples such as those found in swimming pools, spas as well as natural water environments. It may however be advantageously used for determining specific gravity in other aqueous substances such as biological and other fluids containing ionic constituents.
The system is basically a field test but may have utility in analytical laboratories for screening and other applications requiring immediate results. The system comprises a composition and method that in its preferable format utilizes a carrier or matrix for retaining the test reagent and advantageously bringing the active ingredients into contact with the water sample to achieve a calorimetric readout result.
Total Dissolved Solids (TDS) is a term of art used extensively in the water quality area. The term describes itself very aptlyxe2x80x94it is the total amount of inorganic and/or organic substances dissolved in a water sample. It is often described as the xe2x80x9ctotal filterable residuexe2x80x9d of a water sample since it is what remains in the water sample after the suspended or insoluble particulate materials are removed therefrom by a standardized filtration process. Potable and recreational waters usually contain primarily inorganic cationic species such as calcium, magnesium and sodium salts and equivalent amounts of anionic species such as chlorides, sulfates and carbonates.
Historically, TDS has been determined using either gravimetric procedures or estimated by using electrical conductivity measurements. The gravimetric procedure is commonly a laboratory methodology since it usually involves precisely determining the residue remaining (by weight) after drying the sample using standardized drying procedures. Electrical conductivity measurement methods are usually easier to utilize; however, they require a dedicated conductivity meter and commonly require extensive calibration and maintenance practices. They provide estimations of total dissolved solids from the measured conductivity of the dissolved ionic species.
The significance of TDS in determining water quality stems from the fact that high TDS can result in taste problems in the potable water area, and from chemical balance problems in the recreational water area.
More recently, in the medical area, methods have been discovered and developed to measure the specific gravity of body fluids using calorimetric procedures. Generally speaking, the samples being studied in this area are usually body or other biological fluids and have a specific gravity much higher than those found in the water quality area.
The term xe2x80x9cspecific gravityxe2x80x9d is commonly used in the medical area as opposed to the term xe2x80x9ctotal dissolved solidsxe2x80x9d as used in the water quality area. Numerous other terms similar to specific gravity are also used in the medical area. Terms such as xe2x80x9cspecific densityxe2x80x9d, xe2x80x9cionic strengthxe2x80x9d, xe2x80x9cdivalent cation strengthxe2x80x9d, xe2x80x9cosmolalityxe2x80x9d, xe2x80x9cnosmolarityxe2x80x9d, xe2x80x9cion concentrationxe2x80x9d and xe2x80x9cosmotic pressurexe2x80x9d are commonly encountered. Each of these terms has itself a specific chemical meaning and definition but for the purposes of describing medical test systems, the somewhat generic term xe2x80x9cspecific gravityxe2x80x9d will be used. The specific gravity of an aqueous sample can be defined as the ratio of its weight to that of an equal volume of pure water.
The calorimetric analytical schemes used to determine the specific gravity of a body fluid in the medical area basically utilize a polyelectrolyte and an indicator means capable of creating a detectable color response resulting from an ion exchange between the polyelectrolyte and the ions in the aqueous sample. This color response is then correlated to specific gravity.
Polyelectrolytes are usually proprietary polymeric materials having pendant ionic groups. They are well known in the art and are used extensively in chemical ion exchange reactions requiring separation or removal of ionic species.
Human urine is the most common body fluid tested using these analytical schemes to determine specific gravity, and the importance of this test resides in its use to diagnose a situation involving electrolyte imbalance and its associated diseased states.
Prior to the development of these colorimetric methods for determining the specific gravity of body fluids, clinical chemistry methodologies employed cumbersome procedures, and utilized delicate instruments such as refractometers and other specialized devices.
It should be noted here that in addition to having different terminologies, the specific gravity of a body fluid is quite different from the specific gravity or TDS of a water sample. Body fluids such as urine usually have a range of from about 1.005 to 1.030 specific gravity which is equal to a TDS ppm range from 7,000 to 43,000. In contrast, recreational waters typically have a TDS ppm range of values from only 300 to 5,000. Further, potable waters have even lower TDS values of from about 100 to 1,000 ppm.
Because of the different ranges noted above, it has been found that the traditional colorimetric methods used in the medical area were incapable of detecting the very low specific gravity ranges found in the recreational and potable water area.
In contrast to these calorimetric prior art methodologies, it has been found that the present test composition, as will be described later, involves a very different reaction mechanism.
It should also be noted that while the term specific gravity is simply a comparison ratio and has no dimensional tag, the term TDS is usually reported as milligrams per liter (mg/L) or parts per million (ppm).
The prior art surrounding calorimetric specific gravity tests is both extensive and complicated. Most of this prior art involves patent publications and in an attempt to present at least representative patents involving this methodology, the following table is presented:
As stated above the table, this listing is only representative and is being given to present a small window to the plethora of combinations and permutations surrounding the essential components used in current calorimetric specific gravity tests.
The method, composition and device of the present invention involves a highly sensitive reagent system for determining the specific gravity or total dissolved solids of an aqueous sample. The basic system comprises a. a complex of a positively charged polyelectrolyte polymer (PCPPn+) and a negatively charged indicator material (Ixe2x88x92) and b. a buffer system sufficient to maintain the mixture at an exact predetermined pH during the contact and reaction thereof with the aqueous sample. This mixture or composition has been found to generate a color change in a low specific gravity sample depending upon the concentration of anions in the water sample.
To achieve this desired low range detection sensitivity the reagent components were chosen so as to contribute minimally to a background TDS response. A standard color chart or graph is then prepared by using a series of known TDS concentration samples and determining the color produced with the test method or device. Finally, the TDS value of an unknown sample is determined by comparison of the developed color with the standard color chart or instrumentally reading the color in a standardized reflectance calorimeter.